The fabrication of integrated circuits has encountered problems due to submicron precision that must be maintained over a microscopic distance for lithographic processes. In the past, lithographic processes have used electron beam scanning. Electron beam scanning controls the electron beam to serially scan over the surface of a silicon wafer to create the desired image. This process typically takes several hours for the single electron beam to scan the entire wafer during fabrication. Generally, excellent resolution flexibility is possible when using single beam instruments; however, the sequential operation can only scan over a limited area. Thus, to scan the entire wafer, a step and repeat sequence must be used with subsidiary alignment markers being picked up in each frame. Throughout most of the fabrication sequence of a typical integrated circuit, a series of fixed and repetitive exposure patterns are required; hence, the process of aligning and serially scanning is time consuming and ineffective for high volume production.
A high resolution electron image projection tube has been investigated as a means for fabricating large arrays of micron size transistors. See T. W. O'Keefe, J. Vine and R. N. Handy, An Electron Imaging System for the Fabrication of Integrated Circuits. Solid-state Electron 12, 841 (1969). In an electron image projector, pattern details are transferred from a mask onto a silicon wafer using electrons as carriers of the information. Electron image projectors operate by driving electrons from an air stable photocathode which has a surface containing an image of the desired pattern. Subsequently, coaxial electric fields focus the electron image on the silicon wafer targets.
In operation, the electron image projectors are generally arranged as simple diodes with a wafer as the anode and an electron emitting mask as the cathode. In the prior art, ultraviolet radiation has been used in order to excite the mask to emit electrons. Unfortunately, the cost and complexity of tools using this radiation source are excessive and burdensome.
Despite the advancements in electron image projectors, there have thus been problems associated with the method of activating the electrons to be emitted from the mask. Therefore, a need has arisen for an electron image projector which does not require ultraviolet radiation and which has improved quality control characteristics which will permit electron image projectors to emit electrons with reliability and controllability. Additionally, there is a need for an electron emitting mask which is easy to manufacture, simple to operate and which requires little maintenance.